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L292
SWITCH-MODE DRIVER FOR DC MOTORS
DRIVING CAPABILITY : 2 A, 36 V, 30 KHz 2 LOGIC CHIP ENABLE EXTERNAL LOOP GAIN ADJUSTEMENT SINGLE POWER SUPPLY (18 TO 36 V) INPUT SIGNAL SYMMETRIC TO GROUND THERMAL PROTECTION DESCRIPTION The L292 is a monolithic LSI circuit in 15-lead Multiwatt ® package. It is intendedfor use, together with L290 and L291, as a complete 3-chip motor positioning system for applications such as carriage/daisy-wheel position control in type-writes. The L290/1/2 system can be directly controlled by a microprocessor. ABSOLUTE MAXIMUM RATINGS
Symbol Vs Vi Vinhibit Io Ptot Tstg Power Supply Input Voltage Inhibit Voltage Output Current Total Power Dissipation (Tcase = 75 °C) Storage and Junction Temperature Parameter Value 36 - 15 to + Vs 0 to Vs 2.5 25 - 40 to + 150 Unit V V V A W °C
Multiwatt 15
ORDER CODE : L292
TRUTH TABLE
Vinhibit Pin 12 L L H H March 1993 Pin 13 L H L H Disabled Normal Operation Disabled Disabled Output Stage Condition
CONNECTION DIAGRAM (top view)
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L292
THERMAL DATA
Symbol Rth-j-case Parameter Thermal resistance junction-case Max Value 3 Unit °C/W
ELECTRICAL CHARACTERISTICS (Vs = 36 V, Tamb = 25 °C, fosc = 20 KHz unless otherwise specified)
Symbol Vs Id Vos Vinh Parameter Supply Voltage Quiescent Drain Current Input Offset Voltage (pin 6) Inhibit Low Level (pin 12, 13) Inhibit High Level (pin 12, 13) Iinh Low Voltage Condition High Voltage Conditions Ii Vi Input Current (pin 6) Input Voltage (pin 6) Vinh(L) = 0.4 V Vinh(H) = 3.2 V Vl = -8.8 V Vl = +8.8 V Rs1 = Rs2 = 0.2 Io = 2A Io = -2A Io VD Output Current Total Drop Out Voltage Vl = ± 9.8 V Rs1 = Rs2 = 0.2 (inluding sensing resistors) Io = 2 A Io = 1 A VRS Io Vi fosc Sensing Rsistor Voltage Drop Transconductance Tj = 150°C Rs1 = Rs2 =0.2 Rs1 = Rs2 = 0.4 Frequency Range (pin 10) 1 Io = 2 A 205 220 120 30 ±2 5 3.5 0.44 235 9.1 -9.1 3.2 - 100 10 -1.8 0.5 Vs = 20 V (offset null) Io = 0 Test conditions Min. 18 30 Typ. Max. 36 50 ±350 2 Unit V mA mV V V µA µA mA mA V V A V V V mA/V mA/V KHz
BLOCK DIAGRAM AND TEST CIRCUIT
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L292
SYSTEM DESCRIPTION The L290, L291 and L292 are intended to be used as a 3-chip microprocessor controlled positioning system. The device may be used separately - particularly the L292 motor driver - but since they will usually be used together, a description of a typical L290/1/2 system follows. At the time, the microprocessor orders a switch to the position mode, (strobe signal at pin 8 of L291) and within 3 to 4 ms the L292 drives the motor to a null position, where it is held by electronic "detenting". The mechanical/electrical interface consists of an
Figure 1. System Block Diagram
The system operates in two modes to achieve high speed, high-accurancy positioning. Speed commands for the system originate in the microprocessor. It is continuosly updated on the motor position by means of pulses from the L290 tachometer chip, whitch in tur gets its information from the optical encoder. From this basic input, the microprocessor computes a 5-bit control word that sets the system speed dependent on the distance to travel. When the motor is stopped and the microprocessor orders it to a new positio, the system operates initially in an open-loop configuration as there is no feedback from the tachometer generator. A maximum speed is reached, the tachometer chip output backs off the processor signal thus reducing accelering torque. The motor continues to run at rop speed but under closed-loop control. As the target position is approached, the microprocessor lowers the value of the speed-demand word; this reduces the voltage at the main summing point, in effect braking the motor. The braking is applied progressively until the motor is running at minimum speed.
optical encoder which generates two sinusoidal signals 90° out of phase (leading according to the motor direction) and proportional in frequencyto the speed of rotation. The optical encoder also provides an output at one position on the disk which is used to set the initial position. The opto encoder signals, FTA and FTB are filtered by the networks R2C2 and R3 C3 (referring to Fig.4) and are supplied to the FTA/FTB inputs onthe L290. The main function on the L290 is to implement the following expression:
Output signal (TACHO) = dVAB dVAA FTA FTB · · - dt dt | FTA | | FTB |
Thus the mean value of TACHO is proportional to the rotation speed and its polarity indicates the direction of rotation. The above function is performed by amplifying the input signals in A1 and A2 to obtain VAA and VAB (typ.7 Vp). From VAA and VAB the external differentiatior RC networks R5 C6 and R4 C4 give the signals VMA and VMB which are fed to the multipliers.
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The second input to each multipler consists of the sign of the first input of the other multiplier before differentiation,these are obtained using the comparators Cs1 and Cs2. The multiplier outputs, CSA and CSB, are summed by A3 to give the final output signal TACHO. The peak-topeak ripple signal of the TACHOcan be found from the following expression: Vrippl e
p -p
external RC network (R20, C17 - pins 11 and 10) where: 1 fosc = 1 2RC (with R 8.2 K )
=
(2 - 1) 4
·
Vthaco DC
The max value of TACHO is: Vtacho
max
=
2 4
·
Vthaco
DC
Using the coparators C1 and C2 another two signals from VAA and VAB are derived - the logic signalsSTA and STB. This signals are used by the microprocessor to determine the position by counting the pulses. The L2910 internal reference voltage is also derived from VAA and VAB: Vref = | VAA | + | VAB | This reference is used by the D/A converter in the L291 to compensate for variations in input levels, temperature changes and ageing. The "one pulse per rotation" opto encoder output is connected to pin 12 of the L290 (FTF) where it is squared to give the STF logic output for the microprocessor. The TACHO signal and Vref are sent to the L291 via filter networks R8 C8 R9 and R6 C7 R7 respectively. Pin 12 of this chip is the main summing point of the system where TACHO and the D/Aconverter output are compared. The input to D/A converter consists of 5 bit word plus a sign bit supplied by the microprocessor. The sign bit represets the direction of motor rotation. The (analogue) output of the D/A conveter DAC/OUT - is compared with the TACHO signal and the risulting error signal is amplified by the error amplifier, and subsequently appears on pin 1. The ERRV sognal (from pin 1 , L291) is fed to pin 6 of the final chip, the L292 H-bridge motor-driver. This input signals is bidirectional so it must be converted to a positive signal bacause the L292 uses a single supply voltage. This is accomplished by the first stage - the level shifter, which uses an internally generated 8 V reference. This same reference voltage supplies the triangle wave oscillator whose frequency is fixed by the
The oscillator determines the switching frequency of the output stage and should be in the range 1 to 30 KHz. Motor current is regulated by an internal loop in the L292 which is performed by the resistors R18, R19 and the differential current sense amplifier, the output of which is filtered by an external RC network and fed back to the error amplifier. The choise of the external components in these RC network (pins 5, 7, 9) is determined by the motor type and the bandwidth requirements. The values shown in the diagram are for a 5, 5 MH motor. (See L292 Transfer Function Calculation in Application Information). The error signal obtained by the additionof the input and the current feedback signals (pin 7) is used to pulse width modulate the oscillator signal by means of the comparator. The pulse width modulated signal controls the duty cycle of the Hbridge to give an output current corresponding to the L292 input signal. The interval between one side of the bridge switching off and the other switching on, , is programmed by C17 in conjuction with an internal resistor R. This can be foud from: = R
·
Cpin
10.
(C17 in the diagram)
Since R is approximately 1.5 K and the recommended to avoid simultaneous conduction is 2.5 µs Cpin 10 should be around 1.5 nF. The current sense resistors R18 and R19 should be high precision types (maximum tolerance ± 2 %) and the recommended value is given by: Rmax
·
Io
max
0.44 V
It is possible to synchronize two L292 's, if desired, using the network shown in fig. 2. Finally, two enable inputs are provited on the L292 (pins 12 and 13-active low and high respectively). Thus the output stage may be inhibited by taking pin 12 high or by taking pin 13 low. The output will also be inhibited if the supply voltage falls below 18 V.
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L292
The enable inputs were implemented in this way because they are intended to be driven directly by a microprocessor. Currently available microprocessors may generates spikes as high as 1.5 V during power-up. These inputs may be used for a variety of applications such as motor inhibit during reset of the logical system and power-on reset (see fig. 3).
Figure 2.
Figure 3.
Figure 4 . Application Circuit.
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L292
APPLICATION INFORMATION This section has been added in order to help the designer for the best choise of the values of external components. Figure 5. L292 Block Diagram.
The schematic diagram used for the Laplace analysis of the system is shown in fig. 6. Figure 6.
RS1 = RS2 = RS (sensing resistors) 1 = 2.5 · 10-3 (current sensing amplifier transconductance) R4 LM = Motor inductance, RM = Motor resistance, IM = Motor current Gmo =
IM
| s = 0 (DC transfer function from the input of the comparator (VTH) to the motor current (IM)).
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Neglecting the VCEsat of the bridge transistors and the VBE of the diodes: Gmo = 1 RM 2 Vs VR where : VS = supply voltage VR = 8 V (reference voltage) DC TRANSFER FUNCTION In order to be sure that the current loop is stable the following condition is imposed : 1 + sRC = 1 + s LM RM LM RM (pole cancellation) (2) (1)
from which RC =
(Note that in practice R must greater than 5.6 K)
The transfer function is then, IM VI (s) = R2 R4 R1 R3 Gmo 1 + s RF CF Gmo Rs + s R4 C + s2 RF CF R4 C (3)
In DC condition, this is reduced to IM VI (o) = R2 R4 R1 R3
·
1
Rs
=
0.044 A [ ] Rs V
(4)
OPEN-LOOP GAIN AND STABILITY CRITERION For RC = LM / RM , the open loop gain is: A = 1 sR subF C
·
Gmo
Rs
R4
RF Gmo Rs 1 = C 1 + s RF CF R4 s (1 + s RF CF)
(5)
In order to achieve good stability, the phase margin must be greater than 45° when | A | = 1. 1 That means that, at fF = must be | A | < 1 (see fig. 7), that is : 2 RF CF | A | f = Gmo Rs 1 = 2 RF CF R4 C
RF CF < 1 2
(6)
Figure 7. Open Loop Frequency Response
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L292
CLOSED-LOOP SYSTEM STEP RESPONSE a) Small - signals analysis. The transfer function (3) can be written as follows: IM 0.044 (s) = VI Rs 1+ s 2o 2 s + s2 1+ o o 2 (7) Fi gur e 8 . Smal l Si gnal Ste p Respon se (normalized amplitude vs. t / RFCF).
where wo = =
Gmo R s is the cutoff frequency R4 C RF CF
R4 C is the dumping factor 4 RF CF Gmo Rs
By choosing the value, it is possible to determine the system response to an input step signal. Examples : 1) = 1 from which IM (t) = t - t 0.044 [ 1 - e 2RF CF (1 + ) ] · Vi 4 RF CF Rs
V7 = 200 mV/div. IM = 100 mA/div. t = 100 µs/div. with VI = 1.5 Vp.
(where Vi is the amplitude of the input step). 2) = 1 from which 2
IM (t) =
0.044 Rs
(1 - cos
t - t e 2 RF CF ) Vi 2 RF CF
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L292
It is possible to verify that the L292 works in "closedloop" conditions during the entire motor current rise-time: the voltage at pin 7 inverting input of the error amplifier) is locked to the reference voltage VR, present at the non-inverting input of the same amplifier. The previous linear analysis is correct for this example. Descresing the value, the rise-time of the current decreases. But for a good stability, from relationship (6), the maximum value of is: CLOSED LOOP SYSTEM BANDWIDTH. A good choice for x is the value 1 / 2. In this case : 1 + s RF CF IM 0.044 (s) = VI Rs 1 + 2s RF CF + 2s 2 RF 2 CF 2 The module of the transfer function is : | IM 0.044 | = 2 VI Rs (8)
min =
1
2 2
4
(phase margin = 45°)
b) Large signal reponse The large step signal response is limited by slewrate and inductive load. In this case, during the rise-time of the motor current, The L292 works is open-loop condition.
1 + 2 RF 2 CF 2 [ ( 1 + 2 RF CF ) 2 + 1 ] · [ ( 1 - 2 RF CF ) 2 + 1 ]
(9)
IM 0.044 | = 0.707 · (-3 dB), from The cutoff frequency is derived by the expression (9) by putting | VI Rs which : T = 0.9 RF CF
fT =
0.9 2 RF CF
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L292
Example : a) Data
- Motors characteristics:
LM = 5 mH RM = 5 W LM / RM = 1msec - Voltage and current characteristics: Vs = 20 V IM = 2 A - Closed loop bandwidth : 3 kHz - From relationship (4) : Rs = 0.044 VI = 0.2 IM
VI = 9.1 V
b) Calculation
and from (1) : Gmo =
2VS
RM VR
= 1
-1
- RC = 1 msec [from expression (2) ]. - Assuming = 1/ 2 ; from (7) follows :
2 =
1 400 C = 4RF CF · 0.2 2
- The cutoff frequency is : fT = c) Summarising - RC = 1.10-3 sec 1000 C = 1 RF CF 143 · 10-3 = 3 kHz RF CF
- RF CF 47 µs
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C = 47 nF R = 22 K For RF = 510 CF = 92 nF
L292
MULTIWATT15 PACKAGE MECHANICAL DATA
DIM. MIN. A B C D E F G G1 H1 H2 L L1 L2 L3 L4 L7 M M1 S S1 Dia1 21.9 21.7 17.65 17.25 10.3 2.65 4.25 4.63 1.9 1.9 3.65 4.55 5.08 17.5 10.7 22.2 22.1 0.49 0.66 1.02 17.53 19.6 20.2 22.5 22.5 18.1 17.75 10.9 2.9 4.85 5.53 2.6 2.6 3.85 0.862 0.854 0.695 0.679 0.406 0.104 0.167 0.182 0.075 0.075 0.144 0.179 0.200 0.689 0.421 0.874 0.870 1.27 17.78 1 0.55 0.75 1.52 18.03 0.019 0.026 0.040 0.690 0.772 0.795 0.886 0.886 0.713 0.699 0.429 0.114 0.191 0.218 0.102 0.102 0.152 0.050 0.700 mm TYP. MAX. 5 2.65 1.6 0.039 0.022 0.030 0.060 0.710 MIN. inch TYP. MAX. 0.197 0.104 0.063
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L292
Information furnished is believed to be accurate and reliable. However, SGS-THOMSON Microelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of SGS-THOMSON Microelectronics. Specifications mentioned in this publication are subject to change without notice. This publication supersedes and replaces all information previously supplied. SGS-THOMSON Microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of SGS-THOMSON Microelectronics. © 1994 SGS-THOMSON Microelectronics - All Rights Reserved SGS-THOMSON Microelectronics GROUP OF COMPANIES Australia - Brazil - France - Germany - Hong Kong - Italy - Japan - Korea - Malaysia - Malta - Morocco - The Netherlands - Singapore Spain - Sweden - Switzerland - Taiwan - Thaliand - United Kingdom - U.S.A.
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